The invention concerns a heating device fed with free-flowing fuel and connected to an on-board voltage source, in particular, a vehicle heating device with a burner assembly which includes a fuel supply, a combustion air fan and a heating element which ignites a fuel/air mixture after a preheating phase, during which the combustion air fan is switched off or operated at less than the operating blower output, as well as a switching device which automatically turns on or switches the combustion air fan to its operating blower output following completion of the preheating phase. Heating devices of this type are already known (DE-PS Nos. 11 24 377, 11 25 790, and 11 43 725). The duration of the preheating phase in these devices is set. Satisfactory ignition has proven to be problematic in such equipment when the on-board voltage changes. In the case of undervoltage, the temperature on the glowing surface of the heating element, in particular, a glow plug, can be so low that ignition of the fuel/air mixture is no longer guaranteed. Should an overvoltage occur, the heating element, e.g., its heating coil, could burn out.
The invention, thus, has for a principal object the creation of a heating device which produces safe ignition, even when on-board voltage varies, and in which the heating element is simultaneously protected in a simple manner against damage by overvoltage.
This object is achieved, in accordance with a preferred embodiment of the invention, by providing a device of the type mentioned initially with a switching device that is designed as a voltage-dependent time function element, which presets a preheating time that is adjusted based on the magnitude of the on-board voltage.
The invention is based on the fact that the temperature of the heating element is a function of time, i.e., because of the temperature dependence in the heating device, a period of time, which will increase or decrease depending upon the actual voltage, will pass before a predetermined operating temperature is reached. By adjusting the preheating time, the temperature of the heating element can consequently be limited to a value which will prevent overheating of the heating element, even when the heating element is selected in such a way that it ensures that a required ignition temperature is achieved, even at the lower end of the expected on-board voltage range.
In accordance with a further aspect of the invention, the voltage/time performance line of the time-function element is, preferably, adapted to the system of voltage-dependent temperature heating time performance lines of the heating device in such a manner that the temperature reached by the heating device within the given preheating time approximates the maximum allowable temperature of the coil, but does not exceed this temperature. This guarantees a particularly high certainty of ignition being obtained.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, a single embodiment in accordance with the present invention.